1. Field of the Invention
The present invention relates to a control technique of a transfer bias voltage in an image forming apparatus for transferring a toner image onto a sheet by applying the transfer bias voltage to the sheet through a transfer member.
2. Description of the Related Art
Hitherto, the determination of a processing condition of a transfer processing of a toner image onto a sheet has been performed such that a user selects the basis weight of a sheet to be used, a voltage value of a transfer bias voltage is determined based on the information, and transfer onto the sheet is performed. Besides, in order to save the user the trouble of selecting the sheet, after the thickness of the sheet is detected by a thickness detecting sensor or the like, the transfer bias voltage is determined and the transfer processing onto the sheet has been performed.
In general, in sheets of the same material, even if the basis weight or electric resistance is changed, a necessary transfer current is constant. Accordingly, when a transfer bias voltage by which a specified transfer current flows is applied according to the basis weight or electric resistance, an excellent image can be obtained. However, in sheets of different materials, even if the basis weights or electric resistances are almost the same, optimum transfer currents are not always coincident with each other, and poor transfer (poor image) can often occur.
The invention has been made to solve the foregoing problem, and has an object to provide a technique of preventing poor transfer from occurring by suitably controlling a transfer bias voltage according to a processing condition in an image forming apparatus in which a toner image is transferred onto a sheet by applying the transfer bias voltage to the sheet through a transfer member.